Verifying a string is not empty/null in an If condition

Question

I'm writing a script that will accept user input via Read-Host (set to $String), and I want to avoid any issues that could be caused by having a blank value for the variables. Since I'll be using this a lot, I want to implement it into a function that verifies no invalid characters are being used.

I thought I could use an if statement with ![string]::IsNullOrEmpty($String) as one of the conditions:

Function Test-ValidCharacters ($String, $ValidCharacters) {  
  if (($String -match $ValidCharacters) -and (!([string]::IsNullOrEmpty($String)))) {
    return $true
  }
  else {return $false}
}

I also tried this:

Function Test-ValidCharacters ($String, $ValidCharacters) {  
  if (($String -match $ValidCharacters) -and ($String -ceq "")) {
    return $true
  }
  else {return $false}
}

In both of these cases, I can just hit enter when presented with the $String's Read-Host prompt and the script will behave as if the function returned $True (and then later encounter fatal errors). The other half works - if I include characters not specified by $ValidCharacters the function returns $False as expected.

I am sure I'm missing something here. I even tried doing a second nested if statement and got the same result.

Edit: Here's the code snippet where I call the function and notice the issue.

$ValidCharacters = '[^a-zA-Z0-9]'
$FirstN = Read-Host -Prompt "New user's first name"
While (Test-ValidCharacters $FirstN $ValidCharacters -eq $false) {
  Write-Output "$FirstN contains illegal characters. A-Z, a-z, and 0-9 are accepted."
  $FirstN = Read-Host -Prompt "New user's first name"
}

Try If(!([string]::IsNullOrWhitespace($String))){. Also, without knowing what $ValidCharacters is it is really hard to help. — TheMadTechnician
– TheMadTechnician, Commented Jun 15, 2021 at 23:50
You could use [validatescript({$_.Trim() -match '\w'})]$check = Read-Host 'User input here' and avoid the need of the function :) — Santiago Squarzon
– Santiago Squarzon, Commented Jun 15, 2021 at 23:58
It's a range regex - '[a-zA-Z0-9]' - sorry I didn't include it. — Victor Arevalo
– Victor Arevalo, Commented Jun 16, 2021 at 0:04

mklement0 · Accepted Answer · 2021-06-16 03:43:09Z

3

Assuming $ValidCharacters isn't itself an empty string and contains an anchored character-range regex (regular expression) that covers the entire input string, such as ^[a-z0-9./:]+$, given that the -match operator matches any substring by default (note that a better name for the parameter is therefore something like $ValidationRegex):^[1]

In the first function definition, the RHS of your -and operation is redundant - it adds nothing to the conditional, because if $String -match $ValidCharacters is $true, then so is ! [string]::IsNullOrEmpty($String), by definition.
Conversely, in the second function definition your -and operation always returns $false, because $String -ceq "" is by definition $false, if the LHS returned $true.

Assuming that your intent is to prevent empty or all-whitespace input and to ensure that any string - trimmed of incidental leading and/or trailing whitespace - is composed only of expected characters, use the following:

Function Test-ValidCharacters ($String, $ValidCharacters) {
  # Note: no strict need for `return`.
  $String.Trim() -match $ValidCharacters
}

^{[1] Alternatively, stick with $ValidCharacters and pass a regex that describes only a single valid character, such as '[a-z0-9./:]', and construct the entire-string matching regex inside the function with '^' + $ValidCharacters + '+$'}

edited Jun 16, 2021 at 3:43

answered Jun 15, 2021 at 23:51

mklement0

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3 Comments

Victor Arevalo Over a year ago

Yes, the $ValidCharacters input is regex, '[a-zA-Z0-9]' in the test case. I tried your example to see if it behaved any differently, but I could still enter an empty response to Read-Host and it would evaluate as $true. I edited the original post to provide the use case of the function in case that's the cause of the issue.

Santiago Squarzon Over a year ago

@VictorArevalo You're always entering the while loop because: 1. your regex is wrong, instead of '[^a-zA-Z0-9]' it should be '^[a-z0-9]' Note, since match is case insensitive A-Za-z is not needed. thanks mklement0 for reminding of that :) 2. the condition on your while loop should encapsulate the function so PS will validate it first, on the other hand, asking if something is $false doesn't look very clean so for simplicity you can use While (-not (Test-ValidCharacters $FirstN $ValidCharacters)) { .... }

mklement0 Over a year ago

Thanks, @SantiagoSquarzon. My example regex should have been ^[a-z0-9./:]+$ (now fixed), given that -match matches any substring by default.

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